A data center can be modeled as rows of racks that house electronic systems, such as computing systems or other types of electrical devices. The computing systems (such as computers, storage devices, servers, routers, networking devices, etc.) consume power for their operation. The computing systems of the data center may reside in these racks. In a typical data center, there may be dozens or even hundreds of electrical devices. Each of these devices is connected to an electrical power source.
Data centers consume significant amounts of power, much of which is wasted in transmission and distribution, overcooling, and idle servers. Various studies have shown that about 35 W (watts) of electricity goes toward 1 W of actual work. In order to effectively utilize power in data centers and to ultimately reduce the overall power usage, it is critical to first measure how different data center equipment (e.g., servers, routers, storage equipment, cooling unit and power distribution units) consume power, and then to use the consumption information to optimize power usage.
The problem of measuring power consumption in data centers is complex for several reasons. First, the number of devices that consume power can be very large. Any solution will need to be cost effective and will need to apply to large as well as small data centers. Second, data centers use a variety of devices and mechanisms to deliver power. Any solution will need to address the heterogeneity and legacy issues in the data center. Finally, a large number of devices or systems in data centers can be idle or outdated. Some mechanism is needed to enable data center operators to remotely manage data center equipment and the power consumption of data center equipment.
In some cases, data center operators have responded by installing Smart power strips, power distribution strips, or Rack Power Distribution Units (RPDUs) that can measure power at the rack or at the plug level. Some of these RPDUs can use the Internet Protocol (IP) network to transfer this information. Unfortunately, the networking capabilities of many of the conventional RPDUs are not utilized because of the cost and complexity of connecting them to the IP network.
Power in data centers is typically distributed from Uninterruptable Power Systems (UPS) to servers from Power Distribution Units (PDUs) that transform the voltage from 480 VAC to 208/110 VAC to internal electrical distribution panel boards or external power panel boards. The panel boards contain individual electrical circuits of various current or amperage capacities. These electrical power circuits are connected to power distribution strips containing electrical outlets in equipment racks placed on the data center floor to plug the individual servers to meet their power needs.
The power distribution topology provided in the data center may be single path or non-redundant with only one set of UPS. PDUs are installed to provide power to the servers with a single power supply and one plug connection. The topology may also be dual path or redundant whereby multiple UPS systems and two sets of PDUs are installed to provide power from two different electrical systems to each server, which is designed with redundant power supplies that are powered independently from two rack power distribution units or strips (RPDUs). In another data center electrical topology, a primary UPS of a redundant set of UPS systems feeds single PDUs through static transfer switches (STS). If the primary UPS fails, the STS automatically switches the PDU to the alternate back-up source UPS of the redundant set of UPS systems.
Each server's maximum power requirement at full load is required to be provided to the outlet plug in either a single or dual path topology. To meet this requirement each power distribution strip is fed from a PDU electrical circuit that may vary in voltage and power capacity as required by the information technology (IT) equipment to be housed in each equipment rack.
The number and type of servers that may be housed in a specific equipment rack is bound by how much electrical load can be put on each circuit (known as circuit capacity), which typically is between 20-100 amps per circuit. Power is fed from the PDU through the power distribution panel and through a circuit breaker, which is a device that enables a safe means of turning off power being fed from the PDU to the rack power strips for installation and maintenance purposes and to protect the electrical power distribution system from overloading and damage if the server load connected to the circuit exceeds its rated maximum capacity.
New servers are typically assigned to specific racks and circuits based upon the estimated maximum server power requirement and the installed provisioned maximum rated capacity of the circuit breaker.
The typical approach used is as follows:                Look at server faceplate ratings connected to or proposed to be connected to an individual rack power strip, add them up and use this value to see if additional servers can be added to a existing power circuit or to determine the capacity of the circuit breaker that needs to be used.        Another typical method is to consult the server manufacturer's published average usage for the particular server types to be housed in the equipment rack, add them up, and add a safety margin of 20-30% for possible peak usage.        
The problems with both of these approaches are as follows:                Faceplate ratings are based upon the maximum power draw of the server with its maximum resource configuration including central processing units (CPUs) or cores, memory and storage. This maximum power draw is much higher than the typical configuration used in the data center.        The commonly used safety cushion of 20-30% may also be too high or too low depending upon actual peak utilization that is based upon varying application work load demand and diversity factors depending upon what workloads are assigned to the servers housed in the rack.        In both cases, decisions about available capacity may be too conservative or not conservative enough and lead to wasted capacity or unplanned power outages if the installed servers require more power than the installed capacity or in fail-over modes where the sum of the power drawn on the two PDUs is more than the installed UPS capacity.        
As a result of these problems with conventional data center power management systems, the cost of owning and operating a data center can be significantly affected. The implications to the total cost of ownership and operation of the data center can include:                Some available conditioned UPS power is wasted. This wasted power can be expensive.        Reconfiguration of servers in racks may force movement of other servers to other racks or the requirement to statically power cap the servers, which provides a predictable maximum power draw but reduces their performance at peak CPU utilization. In other words, statically determined available capacity does not respond well to changes in server configuration or workload utilization.        Equipment refresh with more energy efficient and significantly more powerful servers, especially for currently operating applications further increases wasted power capacity and lowers average server utilization levels.        